The present invention relates to an activated sludge process and apparatus for effectively removing organic matters, nutrient salts, particulates, etc. dissolved in sewage/waste water by operating activated sludge process of a high biomass concentration combined with a down-flow solid-liquid separation process utilizing a float-concentrated sludge blanket layer. The present invention provides a process for biologically treating sewage/waste water using activated sludge, comprising the steps of (a) floating a sludge by having micro bubbles rise to form a float-concentrated sludge blanket layer and (b) down-flowing mixed liquor, including activated sludge and solids, into the sludge blanket layer, thereby resulting in a solid-liquid separation and an apparatus for operating the process.